1. Field of the Invention
The present invention generally relates to processes for the catalytic partial oxidation of hydrocarbons (e.g., natural gas) to produce a mixture of carbon monoxide and hydrogen (“synthesis gas” or “syngas”).
2. Description of Related Art
The quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming or by autothermal reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.CH4+H2O←→CO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. For many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
Methane residence times in steam reforming are on the order of 0.5-1 second, whereas for heterogeneously catalyzed partial oxidation, the residence time is on the order of a few milliseconds. For the same production capacity, syngas facilities for the partial oxidation of methane can be far smaller, and less expensive, than facilities based on steam reforming. A recent report (M. Fichtner et al. Ind. Eng. Chem. Res. (2001) 40:3475-3483) states that for efficient syngas production, the use of elevated operation pressures of about 2.5 MPa is required. Those authors describe a partial oxidation process in which the exothermic complete oxidation of methane is coupled with the subsequent endothermic reforming reactions (water and CO2 decomposition). This type of process can also be referred to as autothermal reforming or ATR, especially when steam is co-fed with the methane. Certain microstructured rhodium honeycomb catalysts are employed which have the advantage of a smaller pressure drop than beds or porous solids (foams) and which resist the reaction heat of the total oxidation reaction taking place at the catalyst inlet.
The catalytic partial oxidation (“CPOX”) or direct partial oxidation of hydrocarbons (e.g., natural gas or methane) to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+½O2→CO+2H2  (2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol or to fuels. The CPOX reaction is exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes that is possible in a conventional steam reforming process.
While its use is currently limited as an industrial process, the direct partial oxidation or CPOX of methane has recently attracted much attention due to its inherent advantages, such as the fact that due to the significant heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction, in contrast to steam reforming processes. An attempt to overcome some of the disadvantages and costs typical of steam reforming by production of synthesis gas via the catalytic partial oxidation of methane is described in European Patent No. 303,438. According to that method, certain high surface area monoliths coated with metals or metal oxides that are active as oxidation catalysts, e.g., Pd, Pt, Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La, and mixtures thereof, are employed as catalysts. Other suggested coating metals are noble metals and metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic table of the elements.
U.S. Pat. No. 5,149,464 describes a method for selectively converting methane to syngas at 650-950° C. by contacting a methane/oxygen mixture with a solid catalyst which is a d-block transition metal on a refractory support, an oxide of a d-block transition metal, or a compound of the formula MxM′yOz wherein M′ is a d-block transition metal and M is Mg, B, Al, Ga, Si, Ti, Zr, Hf or a lanthanide. U.S. Pat. No. 5,500,149 describes the combination of dry reforming and partial oxidation of methane, in the presence of added CO2 to enhance the selectivity and degree of conversion to synthesis gas. U.S. Pat. No. 5,431,855 demonstrates the catalytic conversion of mixtures of CO2, O2 and CH4 to synthesis gas over selected alumina supported transition metal catalysts. Maximum CO yield reported was 89% at a gas hourly space velocity (GHSV) of 1.5×104 hr−1, temperature of 1,050° K. and pressure of 100 kPa. The addition of CO2 tends to reduce the H2:CO ratio of the synthesis gas, however.
For successful commercial scale operation a catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Dietz III and Schmidt (Catalysis Letters (1995) 33:15-29) describe the effects of 1.4-6 atmospheres pressure on methane conversion and product selectivities in the direct oxidation of methane over a Rh-coated foam monolith. The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors. One of the most important of these factors is the choice of catalyst composition. In most of the existing syngas production processes it is difficult to select a catalyst that will be economical for large scale industrial use, yet will provide the desired level of activity and selectivity for CO and H2 and demonstrate long on-stream life. Moreover, such high conversion and selectivity levels must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort in this field continues to be devoted to the development of catalysts allowing commercial performance without coke formation. Also, in order to overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, and to make possible the operation of the reactor at high gas space velocities, various types of structures for supporting the active catalyst in the reaction zone have been proposed. For example, U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst system having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented in that patent for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat. No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr.−1 to 12,000,000 hr.−1 The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
Vernon, D. F. et al. (Catalysis Letters 6:181-186 (1990)) describe the partial oxidation of methane to synthesis gas using various transition metal catalysts such as Pd, Pt, Ru or Ni on alumina, or certain transition metal oxides including Pr2Ru2O7 and Eu2Ir2O7, under a range of conditions.
U.S. Pat. No. 5,447,705 discloses a catalyst for the partial oxidation of methane having a perovskite crystalline structure and the general composition: LnxA1-yByO3, wherein Ln is a lanthanide and A and B are different metals chosen from Group IVb, Vb, VIb, VIIb or VIII of the Periodic Table of the Elements.
K. L. Hohn and L. D. Schmidt (Applied Catalysis A: General (2001) 211:53-68) describe the effect of space velocity on the partial oxidation of methane using two types of catalyst support geometries. Synthesis gas production by certain rhodium coated monoliths and spheres is discussed, and it is suggested that differences in heat transfer within the two support geometries may play a major role in the different results in catalytic performance observed between spheres and monoliths at increased space velocity. Factors other than chemistry, such as mass and heat transfer within the catalyst region, appear to be important at high flow rates.
PCT Patent Application Publication No. WO 93/01130 describes another catalyst for the production of carbon monoxide from methane. The catalyst is composed of Pd, Pt, Rh or Ir on a pure lanthanide oxide, which may be carried on a ceramic support, preferably zirconia. Pd on Sm2O3 gives relatively low selectivity for either CO or CO2, compared to the selectivities reported for the other compositions evaluated in that study. The methane conversion process is performed with supplied heat, the feed gases comprise very low amount of O2, and very low amounts of H2 are produced as a byproduct of the process.
A. T. Ashcroft, et al. (Nature 344:319-321 (1990)) describe the selective oxidation of methane to synthesis gas using ruthenium-lanthanide containing catalysts. The reaction was carried out at a gas hourly space velocity (GHSV) of 4×104 hr−1 and normal atmospheric pressure. A nitrogen diluent was employed to enhance activity and selectivity.
Lapszewicz, et al. (proceedings of the Symposium on Chemistry and Characterization of Supported Metal Catalysts presented before the Division of Petroleum Chemistry, Inc. 206th National Meeting, American Chemical Society, Chicago, Ill., (Aug. 22-27, 1993) pp. 815-818) describe the use of certain Rh catalysts on pure Sm2O3 and Pt group metals on MgO for catalyzing the partial oxidation of natural gas to syngas. That report focuses on CH4 conversion to carbon monoxide, which reaches a maximum level of 80% using 0.5% Rh on Sm2O3 as the catalyst.
Ruckenstein and Wang (Appl. Catal., A (2000), 198:33-41) describe certain MgO supported Rh catalysts which, at 750° C. and 1 atm, provided a conversion >80% and selectivities of 95-96% to CO and 96-98% to H2, at the high space velocity of 7.2×105 mL/g−1h−1, with very high stability. Those authors report that there was no significant deactivation of the catalyst for up to 96 h of reaction. The strong interactions between rhodium and magnesium oxides are suggested to be responsible for the high stability of the catalyst. In today's syngas production processes, productivity typically falls off when the process is operated at superatmospheric pressure.
Another potential disadvantage of many of the existing catalytic hydrocarbon conversion methods is the need to include steam in the feed mixture to suppress coke formation on the catalyst. Typically, the ratio of steam to methane, or other light hydrocarbon, in the feed gas must be maintained at 1:1 or greater. The volume of gaseous H2O significantly reduces the available reactor space for the production of synthesis gas. Another disadvantage of using steam in the production of syngas is that steam increases the production of CO2, which is carbon that is lost to the process of making CO product. Other existing methods have the potential drawback of requiring the input of a CO2 stream in order to enhance the yield and selectivity of CO and H2 products. Another drawback of some existing processes is that the catalysts that are employed often result in the production of significant quantities of carbon dioxide, steam, and C2+ hydrocarbons. This often renders the product gas mixture unsuitable, for example, for feeding directly into a Fischer-Tropsch type catalytic system for further processing into higher hydrocarbon products. Moreover, for efficient syngas production, the use of elevated operation pressures is necessary in order to ensure the direct transition to a downstream process, such as a Fischer-Tropsch process, without the need for intermediate compression.
At the present time, none of the known processes appear capable of sufficiently high space-time yields. Typically, partial oxidation reactor operation under pressure is problematic because of shifts in equilibrium, undesirable secondary reactions, coking and catalyst instability. Another problem frequently encountered is loss of noble metals due to catalyst instability at higher operating temperatures. Although advancement has been made toward providing higher levels of conversion of reactant gases and better selectivities for CO and H2 reaction products, problems still remain with finding sufficiently stable and long-lived catalysts capable of conversion rates that are attractive for large scale industrial use. Accordingly, a continuing need exists for better processes and catalysts for the production of synthesis gas, particularly from methane or methane containing feeds. In such improved processes the catalysts would be stable at high temperatures and resist coking. They would also retain a high level of conversion activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity and elevated pressures for long periods of time on-stream.